1. Field of the Invention
The invention relates to the provision of regulated DC energization to a load and, more particularly, to the provision of regulated DC energization utilizing high frequency (e.g., 30 Khz) DC to AC to DC conversion with pulse width modulation being used in a degenerative feedback loop to achieve output regulation.
2. Prior Art
A recently developed high speed PWM controller in the form of an eight section IC has been made available for the manufacturer of regulated DC power supplies. The integrated circuit and its internal organization thus suggests the functional configuration of a power supply for which it provides control. In general, the most common source of energization is that available from an AC mains of 50 or 60 Herz, one phase or three phase, at one of several common voltages. The power supply will conventionally contain a first section in which the AC is converted to sustained DC, followed by a DC to AC to DC converter which, in the interests of economics, will operate in the AC stage of the converter above the audible frequency range and below the radio frequencies that are commonly used for communication. A common selection, and one herein employed, is approximately 30 kHz. The regulation which is achieved in power supplies controlled by the foregoing integrated circuit is by means of pulse width modulation of the AC wave form. The process works to readjust the pulse width on a pulse by pulse current mode PWM basis. In the interest of efficiency, the power transistors used for conversion normally operate between "on" and "off" states and produce an output of bi-directional rectangular pulses. Thus the process develops high frequency transients that make an inhospitable environment for a sensitive IC, particularly if the IC is to be energized from energy derived from the DC to DC converter.
In addition, the IC requires an externally provided ramp for comparison with an error quantity to determine the widths of individual pulses in the pulse by pulse current mode modulation process. The usual ramp generated at a capacitor which is charged through a serial resistance from a DC source provides a diminishing slope as time advances from the beginning of the ramp until it is timed out. The result is that the regulation process, where gain depends on the slope of the ramp, loses gain in the higher power limits where gain is most needed.
Power supplies, particularly when used to regulate load current or power supplied to a low voltage load, require a power transformer to match the lower voltage requirement. In the case of a short arc lamp, the lamp may require a run voltage of 20 to 40 volts and the power level may be at the kilowatt levels. The power transformer is thus a necessary item and one of considerable expense, requiring a carefully optimized design. Ideally the transformer should facilitate sensing for output regulation and provide ancillary power for the control circuits.
Finally, in a short arc lamp power supply, the lamp requires in succession 30,000 to 50,000 volts for ignition (Phase I), 100 volts for glow to arc transition (Phase II), and 20 to 40 volts at large currents for run (Phase III) energization. The sensing should accurately reflect the current or power supplied to the arc lamp during run operation and yet be immune to the energization supplied during ignition or transition.